Fluorescent substance and light-emitting device using the same

ABSTRACT

An field effect transistor includes a first semiconductor region, a gate electrode insulatively disposed over the first semiconductor region, source and drain electrodes between which the first semiconductor region is sandwiched, and second semiconductor regions each formed between the first semiconductor region and one of the source and drain electrodes, and having impurity concentration higher than that of the first semiconductor region, the source electrode being offset to the gate electrode in a direction in which the source electrode and the drain electrode are separated from each other with respect to a channel direction, and one of the second semiconductor regions having a thickness not more than a thickness with which the one of second semiconductor regions is completely depleted in the channel direction being in thermal equilibrium with the source electrode therewith.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 12/142,078filed Jun. 19, 2008, which is a continuation of application Ser. No.11/249,946 filed Oct. 13, 2005, which is based upon and claims thebenefit of priority from prior Japanese Patent Application No.2004-303509, filed Oct. 18, 2004, the entire contents of all threeapplications are incorporated herein by reference. Application Ser. No.12/023,677, which is also a continuation of application Ser. No.11/249,946, is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a silicate fluorescent substance to beemployed in a display device, an illuminator or various light sources,and to a light-emitting device employing the silicate fluorescentsubstance.

2. Description of the Related Art

A light-emitting diode (hereinafter referred to as LED) which iscomposed of a combination of a light-emitting element as an excitationlight source and a fluorescent substance is well known. In this LED, itis possible, through variously changing the combination of thelight-emitting element and the fluorescent substance, to realize variousluminescent colors. Among them in particular, in order to realize alight-emitting device which is capable of emitting white light orso-called white LED, there have been proposed various methods such as amethod wherein a light-emitting element which is capable of mainlyemitting blue light and a yellow fluorescent substance are employed as acombination, and a method wherein a light-emitting element which iscapable of emitting near-ultraviolet ray, a blue fluorescent substance,a yellow fluorescent substance and a red fluorescent substance areemployed as a combination. As for the yellow fluorescent substance to beemployed in the white LED, although a YAG fluorescent substance is wellknown, since the YAG fluorescent substance is weak in luminescence whenit is excited with light having a wavelength ranging from 360 nm to 410nm, the employment thereof is limited to an LED where a blue lightsource is employed.

As for the yellow fluorescent substance which is capable of emittingyellow light as an emission spectrum when it is excited with lighthaving a wavelength ranging from 360 nm to 500 nm, there is known asilicate fluorescent substance having a composition represented by:M₂SiO₄:Eu. Among them, although one having a composition of:Ba₂SiO₄:Eu²⁺ is limited in crystal structure to orthorhombic system, onehaving a composition of: Sr₂SiO₄:Eu²⁺ is reported to change its crystalstructure from monoclinic system at a temperature of not more than 85°C. to orthorhombic system at a temperature higher than 85° C.

The luminescence of the orthorhombic system of Ba₂SiO₄:Eu²⁺ and of themonoclinic system of Sr₂SiO₄:Eu²⁺ would be greenish yellow having a peakwavelength of 520-540 nm or so. When a portion of Sr of Sr₂SiO₄:Eu²⁺ isreplaced by Ba, it would become possible to obtain the orthorhombicsystem even at room temperature and the luminescence thereof wouldbecome yellow of high color purity having a peak wavelength of 570 nm.Therefore, in order to obtain yellow luminescence of high color purityin a silicate fluorescent substance represented by a composition ofM₂SiO₄:Eu, it is indispensable to include Ba. Further, in order toobtain an emission band having a desirable wavelength within the rangeof 520 to 600 nm, it is required to adjust the composition offluorescent substance through the control of the content of Ba.

However, since a Ba compound has a bad influence on human body, theemployment thereof is restricted by the toxic substance controlregulations. Therefore, even if the fluorescent substance containing Bacompound is capable of emitting yellow luminescence of high colorpurity, the content of Ba compound should preferably be limited asminimal as possible.

BRIEF SUMMARY OF THE INVENTION

A fluorescent substance according to one aspect of the present inventioncomprises an alkaline earth ortho-silicate, the fluorescent substancebeing activated by Eu²⁺, and further comprising at least one selectedfrom the group consisting of La, Gd, Cs and K.

A light-emitting device according to another aspect of the presentinvention comprises a light-emitting element capable of emitting lightat wavelength of 360 nm to 500 nm, and a layer of fluorescent substanceformed on the light-emitting element, the fluorescent substancecomprising an alkaline earth ortho-silicate, the fluorescent substancebeing activated by Eu²⁺, and further comprising at least one selectedfrom the group consisting of La, Gd, Cs and K.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a cross-sectional view schematically illustrating thestructure of a light-emitting device according to one embodiment of thepresent invention;

FIG. 2 is a graph illustrating an excitation spectra of the fluorescentsubstances of Example 1 and Comparative Example 2;

FIG. 3 is a graph illustrating an emission spectrum of the fluorescentsubstance of Example 1;

FIG. 4 is a graph illustrating an X-ray diffraction pattern of thefluorescent substance of Example 1;

FIG. 5 is a graph illustrating an X-ray diffraction pattern of theconventional Sr₂SiO₄:Eu (monoclinic system);

FIG. 6 is a graph illustrating an X-ray diffraction pattern of theconventional (Sr, Ba)₂SiO₄:Eu (orthorhombic system);

FIG. 7 is a graph illustrating an emission spectrum of the fluorescentsubstance of Example 2;

FIG. 8 is a graph illustrating an emission spectrum of the fluorescentsubstance of Example 3;

FIG. 9 is a graph illustrating an emission spectrum of the fluorescentsubstance of Example 4;

FIG. 10 is a graph illustrating an emission spectrum of a white LEDcomprising a combination of the fluorescent substance of Example 1 andan LED chip exhibiting an emission peak wavelength of 470 nm;

FIG. 11 is a graph illustrating an emission spectrum of the fluorescentsubstance of Comparative Example 1; and

FIG. 12 is a graph illustrating an emission spectrum of the fluorescentsubstance of Comparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Next, embodiments of the present invention will be illustrated.

As a result of extensive studies made by the present inventors, it hasbeen found out that in the case of a fluorescent substance made ofalkaline earth ortho-silicate and activated by Eu²⁺, there are elementswhich are capable of changing the crystal structure of the fluorescentsubstance from monoclinic system to orthorhombic system in the samemanner as Ba. As such elements, it is possible to employ at least oneselected from La, Gd, Cs and K. These elements are substantiallynon-poisonous to human body. The present invention has been accomplishedbased on the aforementioned findings.

The fluorescent substance made of Ba-containing alkaline earthortho-silicate and activated by Eu²⁺ can be represented by the followinggeneral formula (1).

(Sr,Ca,Ba,Eu)₂SiO₄   (1)

The fluorescent substance according to one embodiment of the presentinvention can be represented by the following general formula (2) forinstance.

(Sr_(1-x-y-z-w)Ca_(x)Ba_(y)A_(z)Eu_(w))₂Si_(v)O_(2+2v)   (2)

(in the general formula (2), A is at least one selected from the groupconsisting of La, Gd, Cs and K; and x, y, z, w and v are numeric valuessatisfying the following relationships: 0≦x≦0.8; 0≦y≦0.6; 0<z≦0.1;0.001≦w≦0.2; 0<(1-x-y-z-w)<1; and 0.9≦v≦1.1)

If the content of Ca is excessive, the emission efficiency offluorescent substance would be degraded. However, if the content (x) ofCa is limited to not more than 0.8, such degradation can be avoided.Further, when the influence of Ba on human body is taken into account,the content of Ba should preferably be as minimal as possible. Althoughit is most preferable that y=0, Ba may be included to a certain extentin order to adjust the emission wavelength. In this case, as long as yis confined to not more than 0.6, the influence of poisonous Ba can beminimized.

The element A other than alkaline earth metal elements is at least oneselected from La, Gd, Cs and K. These elements may be employed singly orin combination of two or more. If the content of these elements is toolarge, it will lead to the generation of hetero-phase, thus degradingthe emission efficiency of fluorescent substance. Thus, when the numericvalue of z is confined to not more than 0.1, the aforementioned problemwould be overcome.

As shown in the aforementioned general formula (2), the fluorescentsubstance according to the embodiment of the present invention is asilicate compound activated by Eu²⁺. The compositional ratio w of Eushould preferably be confined to the range of 0.001 to 0.2. If w is lessthan 0.001, it may become difficult to secure a sufficient luminanceintensity. On the other hand, if w is more than 0.2, the luminanceintensity of fluorescent substance may be degraded due to concentrationquenching.

Further, the compositional ratio v of Si should preferably be confinedto the range of 0.9 to 1.1. If v is less than 0.9, a second emissionband having a wavelength of nearly 500 nm generates, thus leading to thespreading of emission spectrum. On the other hand, if v is more than1.1, it may become difficult to secure a sufficient luminance intensity.Therefore, v should preferably be confined within the range of 0.95 to1.05.

The fluorescent substance according to one embodiment of the presentinvention can be synthesized according to the following procedure.

First of all, the oxide powder of each of constituent elements isweighed to obtain a predetermined quantity thereof. Then, by ball mill,etc., oxide powders of these constituent elements thus weighed are mixedwith each other together with a suitable quantity of ammonium chlorideemployed as a crystal growth-promoting agent. It is also possible toemploy various compounds which can be turned into oxides through thethermal decomposition thereof. For example, it is possible to employEu₂O₃, etc. as a raw material for Eu; CaCO₃, etc. as a raw material forCa; SrCO₃, etc. as a raw material for Sr; BaCO₃, etc. as a raw materialfor Ba; La₂O₃, etc. as a raw material for La; Gd₂O₃, etc. as a rawmaterial for Gd; CsCl, etc. as a raw material for Cs; KCl, etc. as a rawmaterial for K; and SiO₂, etc. as a raw material for Si. If any of theseraw material powders are capable of acting as a crystal growth-promotingagent, an additional crystal growth-promoting agent such as ammoniumchloride may not necessarily be separately incorporated into a mixtureof the aforementioned oxide powders.

As for the crystal growth-promoting agent, it is possible to employ,other than ammonium chloride, chlorides, bromides or iodides ofammonium, alkaline metal or alkaline earth metal. In order to preventany substantial increase in hygroscopicity of fluorescent substance, thecontent of crystal growth-promoting agent should preferably be confinedto the range of about 0.5 to 30% by weight based on an entire quantityof raw material powders.

Thereafter, a mixture of raw materials is placed in a crucible andpre-fired (pre-baked) for 1 to 3 hours in air atmosphere at atemperature ranging from 500 to 700° C. The fired (baked) material thusobtained is further subjected to mixing and then firing (baking) for 3to 7 hours in a reducing atmosphere consisting of a mixed gas of N₂/H₂at a temperature ranging from 1000 to 1600° C. A first fired productthus obtained is pulverized and again placed into a vessel. With respectto the degree of pulverization, there is not any particular limitation.Namely, the pulverization can be performed by mortar to such an extentthat agglomerates that have been generated due to the firing can bepulverized to increase the surface area of the fired powder.

The pulverized powder is again placed in a furnace, which is then purgedwith nitrogen gas in vacuum. The vacuum on this occasion shouldpreferably be 1000 Pa or less. If the degree of vacuum is higher than1000 Pa, the water adhered onto the raw material cannot be removed.

Then, the aforementioned first fired product is further subjected tofiring for 2 to 6 hours in a reducing atmosphere consisting of N₂/H₂ andhaving a hydrogen concentration of 5% to 100% at a temperature rangingfrom 1000 to 1600° C. The fired product thus obtained is pulverized bymortar and sieved using a sieve of suitable mesh size, thereby obtaininga fluorescent particle consisting of an alkaline earth ortho-silicaterepresented by the aforementioned general formula (2).

The fluorescent particle according to one embodiment of the presentinvention may provide with a surface-covering material on its surface.The covering material can be formed of at least one selected from thegroup consisting of silicone resin, epoxy resin, fluorinated resin,tetraethoxy silane (TEOS), silica, zinc silicate, aluminum silicate,calcium polyphosphate, silicone oil, and silicone grease. It ispossible, with the employment of this covering material, to provide thefluorescent substance with moisture-preventing properties. The zincsilicate and aluminum silicate can be represented for example byZnO-aSiO₂ (1≦a≦4), Al₂O₃-bSiO₂ (1≦b≦10), respectively. The surface ofthe fluorescent particle may not necessarily be completely covered withthe covering material but the surface of the fluorescent particle may bepartially exposed. As long as the covering material consisting of any ofthe aforementioned materials is existed on the surface of thefluorescent particle, it is possible to obtain the moisture-preventingproperties.

The covering material can be applied to the surface of fluorescentparticle by using a dispersion or solution thereof. Specifically, thefluorescent particle is immersed in a dispersion or solution of thecovering material for a prescribed period of time and then dried byheating, etc. to deposit the covering material on the surface offluorescent particle. In order to secure the effects of the coveringmaterial without badly affecting the inherent properties of fluorescentsubstance, the ratio of the covering material should preferably beconfined within the range of about 0.1 to 50% by volume based on thefluorescent substance.

FIG. 1 shows a cross-sectional view schematically illustrating alight-emitting device according to one embodiment of the presentinvention.

In the light-emitting device shown in FIG. 1, a resinous stem 200comprises a pair of leads 201 and 202 forming a lead frame, and a resinportion 203 formed integral with the lead frame. The resin portion 203includes a recess 205 having an upper opening, an area of which is madelarger than that of the bottom thereof. The inner wall of this recess205 is formed into a light-reflection surface 204.

On a central portion of the circular bottom of recess 205, there ismounted a light-emitting chip 206 by a Ag paste, etc. As for thelight-emitting chip 206, it is possible to employ one emittingultraviolet luminescence or one emitting luminescence of visible lightrange. For example, it is possible to employ GaAs-based or GaN-basedsemiconductor light-emitting elements. The electrodes (not shown) oflight-emitting chip 206 are connected, through a bonding wire 207 and abonding wire 208, with a lead 201 and a lead 202, respectively.Incidentally, the arrangement of the lead 201 and the lead 202 can beoptionally altered.

A fluorescent layer 209 is disposed in the recess 205 of resin portion203. This fluorescent layer 209 can be formed by dispersing afluorescent substance 210 proposed by one embodiment of the presentinvention into a resin layer 211 made of silicone resin for example atan amount ranging from 5 to 50% by weight.

As for the light-emitting chip 206, it is possible to employ those offlip chip type where an n-type electrode and a p-type electrode aredisposed on the same surface. In this case, it is possible to preventthe disconnection or peeling of wire or to overcome problems originatingfrom wire such as photoabsorption by wire, thereby making it possible toobtain a semiconductor light-emitting device which is excellent inreliability and luminescence intensity. Further, by using an n-typesubstrate for the manufacture of the light-emitting chip 206, thefollowing structure can be created. More specifically, an n-typeelectrode is formed on the rear surface of an n-type substrate, a p-typeelectrode is formed on the upper surface of semiconductor layer on thesubstrate, and the n-type electrode or the p-type electrode is mountedon one of leads. Further, the p-type electrode lead or the n-typeelectrode is connected via wire with the other lead.

The size of the light-emitting chip 206 and the size and configurationof the recess 205 can be optionally altered. In the case of fluorescentsubstance according to one embodiment of the present invention, when itis excited by a light having a wavelength ranging from 360 nm to 500 nm,it will emit luminescence of green-yellow-orange colors having awavelength ranging from 520 nm to 600 nm. In other words, thefluorescent substance according to one embodiment of the presentinvention emits luminescence of colors ranging from green to orange.More specifically, the emission spectrum of the fluorescent substanceaccording to one embodiment of the present invention includes a singleemission band having a wavelength falling between 520 nm and 600 nm.Incidentally, by the expression of “a single emission band”, it meansthat the spectrum has a single emission peak or a band structure whichis free from a shoulder-like up and down. When the fluorescent substanceaccording to one embodiment of the present invention is employed incombination with a blue-emitting fluorescent substance and ared-emitting fluorescent substance, it will be also possible to obtainwhite beam.

Since the fluorescent substance according to one embodiment of thepresent invention has a orthorhombic structure, it is possible, throughthe fluorescent layer containing this fluorescent substance, to obtain alight-emitting device which is capable of emitting luminescenceexcellent in color purity.

The present invention will be further explained in detail with referenceto examples and comparative examples. However, it should not beconstrued that the present invention is limited to these examples.

EXAMPLE 1

First of all, a fluorescent substance having a composition representedby (Sr_(0.915)La_(0.06)Eu_(0.025))₂SiO₄ was prepared. As raw materialpowders, SrCO₃ powder, La₂O₃ powder, Eu₂O₃ powder and SiO₂ powder wereprepared and weighed to obtain a prescribed quantity of mixed powders.As a crystal growth-promoting agent, NH₄Cl was added to the mixedpowders at an amount of 1.5% by weight based on a total quantity of themixed powders and the resultant mixture was homogeneously mixed togetherby ball mill.

The mixed raw material thus obtained was introduced into a liddedalumina crucible and pre-fired for one hour in air atmosphere at atemperature of 600° C., thereby decomposing NH₄Cl. Then, the firedmaterial thus obtained was further subjected to firing for 3 to 7 hoursin a reducing atmosphere consisting of a mixed gas of N₂/H₂ at atemperature ranging from 1000 to 1600° C. to obtain a first firedproduct. The first fired product thus obtained was pulverized and againplaced into the crucible, which was then placed in a furnace. Thefurnace was then purged with nitrogen gas in vacuum. Further, thepulverized powder was further subjected to firing for 2 to 6 hours in areducing atmosphere consisting of N₂/H₂ and having a hydrogenconcentration of 5% to 100% at a temperature ranging from 1000 to 1600°C. to obtain a second fired product. This fired product was pulverizedby mortar and sieved using a sieve having a mesh aperture of 75 μm,thereby obtaining the fluorescent substance of Example 1.

The excitation spectrum of this fluorescent substance is shown as acurve “a” in FIG. 2. In FIG. 2, the curve “b” shows the excitationspectrum of the fluorescent substance of Comparative Example 2. It willbe seen from the curve “a” that the fluorescent substance of thisexample can be excited in a wide wavelength ranging from UV to yellow inthe same manner as the fluorescent substance of Comparative Example 2.

FIGS. 3 and 4 show the emission spectrum obtained as the fluorescentsubstance of this example was excited at a wavelength of 395 nm and anX-ray diffraction pattern of the fluorescent substance of this example,respectively. The emission spectrum shown herein was obtained bymeasuring the emission spectrum of the fluorescent substance by using aninstantaneous multi photometric system (IMUC-7000G type; Ohtsuka DenshiCo., Ltd.) as the fluorescent substance was excited by a light-emittingdiode having peak wavelengths of 395 nm and 470 nm. As indicated by theemission spectrum of FIG. 3, the spectrum has a single emission peak andis free from shoulder-like up and down. In view of these facts, it willbe seen that the emission spectrum to be obtained as the fluorescentsubstance of this example is excited by a light having a wavelengthranging from 360 nm to 500 nm will have a single peak in a wavelengthranging from 520 nm to 600 nm. Further, the X-ray diffraction patternagrees with a JCPDS (Joint committee on Powder Diffraction Standards)card 39-1256. Incidentally, this JCPDS card is related with the database of powder X-ray diffraction and compiled and issued based on JCPDS.Since there is no peak in the vicinity of 2θ=34.2°, the fluorescentsubstance of this example was of orthorhombic system in crystalstructure as clearly shown in the X-ray diffraction pattern of FIG. 4.

For reference, the X-ray diffraction pattern of the conventionalSr₂SiO₄:Eu fluorescent substance is shown in FIG. 5. This diffractionpattern agrees with JCPDS card 76-1630 and has a peak in the vicinity of2θ=34.2°, and hence this fluorescent substance is of monoclinic systemin crystal structure. Further, FIG. 6 shows the X-ray diffractionpattern of the conventional (Sr,Ba)₂SiO₄:Eu fluorescent substance. Asshown in the X-ray diffraction pattern of FIG. 6, this diffractionpattern agrees with JCPDS card 39-1256 and there is no peak in thevicinity of 2θ=34.2°, thus indicating that the conventional(Sr,Ba)₂SiO₄:Eu fluorescent substance is of orthorhombic system incrystal structure. From the comparison with the X-ray diffractionpattern of FIG. 4, it will be clearly recognized that the fluorescentsubstance of the embodiment of the present invention is of orthorhombicsystem in crystal structure as in the case of the conventionalfluorescent substance containing Ba.

EXAMPLE 2

First of all, a fluorescent substance having a composition representedby (Sr_(0.915)Gd_(0.060)Eu_(0.025))₂SiO₄ was prepared. As raw materialpowders, SrCO₃ powder, Gd₂O₃ powder, Eu₂O₃ powder and SiO₂ powder wereprepared and weighed to obtain a prescribed quantity of mixed powders.As a crystal growth-promoting agent, NH₄Cl was added to the mixedpowders at an amount of 1.5% by weight based on a total quantity of themixed powders and the resultant mixture was homogeneously mixed togetherby ball mill.

Thereafter, the fluorescent substance of Example 2 was obtained byfollowing the same manufacturing method as described in Example 1. Theemission spectrum of this fluorescent substance as it was excited at awavelength of 395 nm is shown in FIG. 7. As indicated by the emissionspectrum of FIG. 7, the emission spectrum obtained as the fluorescentsubstance of this example was excited by a light having a wavelengthranging from 360 nm to 500 nm had a single emission band in a wavelengthranging from 520 nm to 600 nm.

EXAMPLE 3

First of all, a fluorescent substance having a composition representedby (Sr_(0.920)K_(0.055)Eu_(0.025))₂SiO₄ was prepared. As raw materialpowders, SrCO₃ powder, KCl powder, Eu₂O₃ powder and SiO₂ powder wereprepared and weighed to obtain a prescribed quantity of mixed powders.These mixed powders were homogeneously mixed together by ball mill. Inthis example, since the KCl employed as a raw material for K was capableof acting as a crystal growth-promoting agent, NH₄Cl was not added.

Thereafter, the fluorescent substance of Example 3 was obtained byfollowing the same manufacturing method as described in Example 1. Theemission spectrum of this fluorescent substance as it was excited at awavelength of 395 nm is shown in FIG. 8. As indicated by the emissionspectrum of FIG. 8, the emission spectrum obtained as the fluorescentsubstance of this example was excited by a light having a wavelengthranging from 360 nm to 500 nm had a single emission band in a wavelengthranging from 520 nm to 600 nm.

EXAMPLE 4

First of all, a fluorescent substance having a composition representedby (Sr_(0.915)Cs_(0.030)La_(0.030)Eu_(0.025))₂SiO₄ was prepared. As rawmaterial powders, SrCO₃ powder, CsCl powder, La₂O₃ powder, Eu₂O₃ powderand SiO₂ powder were prepared and weighed to obtain a prescribedquantity of mixed powders. These mixed powders were homogeneously mixedtogether by ball mill. In this example, although the CsCl employed as araw material for Cs was capable of acting as a crystal growth-promotingagent, NH₄Cl was also added in order to adjust the ratio of Cl.

Thereafter, the fluorescent substance of Example 4 was obtained byfollowing the same manufacturing method as described in Example 1. Theemission spectrum of this fluorescent substance as it was excited at awavelength of 395 nm is shown in FIG. 9. As indicated by the emissionspectrum of FIG. 9, the emission spectrum obtained as the fluorescentsubstance of this example was excited by a light having a wavelengthranging from 360 nm to 500 nm had a single emission band in a wavelengthranging from 520 nm to 600 nm.

EXAMPLE 5

First of all, a fluorescent substance having a composition representedby (Sr_(0.915)Ba_(0.030)La_(0.030)Eu_(0.025))₂SiO₄ was prepared. As rawmaterial powders, SrCO₃ powder, BaCO₃ powder, La₂O₃ powder, Eu₂O₃ powderand SiO₂ powder were prepared and weighed to obtain a prescribedquantity of mixed powders, which were then homogenously mixed by ballmill. As a crystal growth-promoting agent, NH₄Cl was added to the mixedpowders at an amount of 1.5% by weight based on a total quantity of themixed powders and the resultant mixture was homogeneously mixed togetherby ball mill.

Thereafter, the fluorescent substance of Example 5 was obtained byfollowing the same manufacturing method as described in Example 1. Theemission spectrum of this fluorescent substance as it was excited at awavelength of 395 nm indicated a peak wavelength of 575 nm, and theemission spectrum as this fluorescent substance was excited at awavelength of 470 nm indicated a peak wavelength of 580 nm. Just likethe fluorescent substances of Examples 1 to 4, the emission spectrumobtained as the fluorescent substance of this example was excited by alight having a wavelength ranging from 360 nm to 500 nm had a singleemission band in a wavelength ranging from 520 nm to 600 nm.

From the results of X-ray diffraction, it was possible to confirm thatthe fluorescent substances of Examples 2 to 5 also were of orthorhombicsystem in crystal structure as in the case of Example 1.

EXAMPLE 6 LED

The fluorescent substance of Example 1, a blue fluorescent substance anda red fluorescent substance were dispersed in epoxy resin to prepare aresinous mixture. Incidentally, as for the blue fluorescent substance,an europium-activated alkaline earth chlorophosphate fluorescentsubstance was employed, and as for the red fluorescent substance, aneuropium-activated lanthanum oxysulfide fluorescent substance wasemployed. This resinous mixture was coated on an LED package where anLED chip exhibiting an emission peak wavelength of 395 nm was mountedthereon, thereby obtaining a light-emitting device as shown in FIG. 1.

It was confirmed that the light-emitting device thus obtained wascapable of emitting a white beam excellent in color renderingproperties.

EXAMPLE 7 LED

The fluorescent substance of Example 1 was dispersed in epoxy resin toprepare a resinous mixture. This resinous mixture was coated on an LEDpackage where an LED chip exhibiting an emission peak wavelength of 470nm was mounted thereon, thus manufacturing a white LED.

The emission spectrum of the white LED thus obtained is shown in FIG.10. The luminescence color in this case was: x=0.337 and y=0.303 inchromaticity, and the color temperature was 5247K.

COMPARATIVE EXAMPLE 1

First of all, a fluorescent substance having a composition representedby (Sr_(0.285)Ba_(0.665)Eu_(0.050))₂SiO₄ was prepared. As raw materialpowders, SrCO₃ powder, BaCO₃ powder, Eu₂O₃ powder and SiO₂ powder wereprepared and weighed to obtain a prescribed quantity of mixed powders.As a crystal growth-promoting agent, NH₄Cl was added to the mixedpowders at an amount of 1.5% by weight based on a total quantity of themixed powders and the resultant mixture was homogeneously mixed togetherby ball mill.

Thereafter, the fluorescent substance of Comparative Example 1 wasobtained by following the same manufacturing method as described inExample 1. The emission spectrum of this fluorescent substance as it wasexcited at a wavelength of 395 nm is shown in FIG. 11. As indicated bythe emission spectrum of FIG. 11, the emission spectrum obtained wasgreen luminescence having a single emission band at a wavelength of 525nm. However, since the fluorescent substance of this comparative examplecontained Ba at compositional ratio of more than 0.6, it was impossibleto obviate the problem of toxicity.

COMPARATIVE EXAMPLE 2

First of all, a fluorescent substance having a composition representedby (Sr_(0.915)Ba_(0.060)Eu_(0.025))₂SiO₄ was prepared. As raw materialpowders, SrCO₃ powder, BaCO₃ powder, Eu₂O₃ powder and SiO₂ powder wereprepared and weighed to obtain a prescribed quantity of mixed powders.As a crystal growth-promoting agent, NH₄Cl was added to the mixedpowders at an amount of 1.5% by weight based on a total quantity of themixed powders and the resultant mixture was homogeneously mixed togetherby ball mill.

Thereafter, the fluorescent substance of Comparative Example 2 wasmanufactured in the same manner as described in Example 1.

The emission spectrum of this fluorescent substance as it was excited ata wavelength of 395 nm is shown in FIG. 12. As indicated by the emissionspectrum of FIG. 12, since the fluorescent substance of ComparativeExample 2 contained Ba, the emission spectrum obtained as thefluorescent substance of this comparative example was excited by a lighthaving a wavelength ranging from 360 nm to 500 nm had a single emissionband in a wavelength ranging from 520 nm to 600 nm.

On the other hand, in the fluorescent substances according to Examples 1to 4, even if Ba was not included in the compositions thereof, it waspossible to exhibit an emission band having the same features as thiscomparative example. In the case of the fluorescent substance of Example5, although it was possible to exhibit an emission band having the samefeatures as Comparative Example 2, the quantity of Ba employed thereinwas a half of the quantity of Ba of Comparative Example 2.

In the following Table 1, the compositions of aforementioned Examplesand Comparative Examples, emission peak and the types of crystal aresummarized.

TABLE 1 Peak emission wavelength (nm) Composition (mole ratio) 395 nm470 nm Crystal Composition Sr Ba A1 A2 Eu Si Excitation Excitation typeExample 1 (Sr,A1)₂SiO₄:Eu 1.83 — La 0.12 — — 0.05 1.00 575 580Orthorhombic system 2 (Sr,A1)₂SiO₄:Eu 1.83 — Gd 0.12 — — 0.05 1.00 570585 Orthorhombic system 3 (Sr,A1)₂SiO₄:Eu 1.84 — K 0.11 — — 0.05 1.00565 580 Orthorhombic system 4 (Sr,A1,A2)₂SiO₄:Eu 1.83 — Cs 0.06 La 0.060.05 1.00 575 580 Orthorhombic system Example 5 1.83 0.060 La 0.06 — —0.05 1.00 575 580 Orthorhombic system Comp. 1 Sr₂SiO₄:Eu 1.95 — — — — —0.05 1.00 540 535 Monoclinic Ex. system 2 (Sr,Ba)₂SiO₄:Eu 1.83 0.12  — —— — 0.05 1.00 570 575 Orthorhombic system

As shown in Table 1, the fluorescent substances according to theembodiments of the present invention where at least part of Ba isreplaced by at least one selected from La, Gd, Cs and K were all formedof orthorhombic system in crystal structure. As a result, when thesefluorescent substances are excited by a light having a wavelength of 395nm or 475 nm, a single emission band generates in a wavelength rangingfrom 520 nm to 600 nm, thus enabling these fluorescent substances toemit luminescence excellent in color purity.

It is clear that these advantages are by no means inferior to those ofthe conventional Ba-containing fluorescent substance of ComparativeExample 2.

According to the present invention, it is possible to provide a silicatefluorescent substance which is capable of emitting luminescence ofgreen-yellow-orange colors excellent in color purity and having a singleemission band in a wavelength ranging from 520 nm to 600 nm as it isexcited by a light having a wavelength ranging from 360 nm to 500 nm.Further, according to the present invention, it is possible to provide alight-emitting device where this silicate fluorescent substance isemployed.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A method for manufacturing a fluorescent substance comprising: mixinga raw material for Eu, a raw material for Si, at least one raw materialpowder of alkaline earth, and at least one selected from the groupconsisting of raw materials for La, Gd, Cs, and K to obtain a mixture ofraw materials; pre-firing the mixture for 1 to 3 hours in an airatmosphere at a temperature ranging from 500 to 700° C. to obtain abaked material; mixing the baked material and firing for 3 to 7 hours ina reducing atmosphere consisting of a mixed gas of N₂/H₂ at atemperature ranging from 1000 to 1600° C. to obtain a first firedproduct; pulverizing the first fired product to obtain a pulverizedfirst fired product; firing the pulverized first fired product for 2 to6 hours in a reducing atmosphere consisting of N₂/H₂ and having ahydrogen concentration of 5% to 100% at a temperature ranging from 1000to 1600° C. to obtain a fluorescent substance consisting of an alkalineearth ortho-silicate; pulverizing the fluorescent substance to obtain afluorescent particle; sieving the fluorescent particle; and providing asurface-covering material on a surface of the fluorescent particle. 2.The method according to claim 1, wherein the covering material isapplied to the surface of the fluorescent particle by using a dispersionor solution of the covering material.
 3. The method according to claim2, wherein the fluorescent particle is immersed in the dispersion orsolution for a prescribed period of time and then dried by heating todeposit the covering material on the surface of the fluorescentparticle.
 4. The method according to claim 1, wherein thesurface-covering material is formed of at least one selected from thegroup consisting of silicone resin, epoxy resin, fluorinated resin,tetraethoxy silane, silica, zinc silicate, aluminum silicate, calciumpolyphosphate, silicon oil, and silcone grease.
 5. The method accordingto claim 1, wherein the covering material is provided at a ratio of 0.1to 50% by volume based on the volume of the fluorescent substance. 6.The method according to claim 1, further comprising before firing thepulverized first fired product, placing the pulverized first firedproduct into a furnace; and purging the furnace with nitrogen gas invacuum
 7. The method according to claim 6, wherein the pulverized firstfired product housed into a vessel before placed in the furnace.
 8. Themethod according to claim 1, wherein the raw material for Eu is Eu₂O₃powder.
 9. The method according to claim 1, wherein the raw material forSi is SiO₂ powder.
 10. The method according to claim 1, wherein the rawmaterial powder of alkaline earth comprises SrCO₃ powder.
 11. The methodaccording to claim 1, wherein the raw material powder of alkaline earthfurther comprises CaCO₃ powder.
 12. The method according to claim 1,wherein the raw material for La is La₂O₃ powder.
 13. The methodaccording to claim 1, wherein the raw material for Gd is Gd₂O₃ powder.14. The method according to claim 1, wherein the raw material for Cs isCsCl powder.
 15. The method according to claim 1, wherein the rawmaterial for K is KCl powder.
 16. The method according to claim 1,further comprising adding a crystal growth-promoting agent to themixture of raw materials.
 17. The method according to claim 16, whereina content of the crystal growth-promoting agent is 0.5-30% by weightbased on an entire quantity of raw material powders.
 18. The methodaccording to claim 16, wherein the crystal growth-promoting agent isselected from the group consisting of ammonium chloride, ammoniumbromide, ammonium iodide, chloride of alkaline metal, bromide ofalkaline metal, iodide of alkaline metal, chloride of alkaline earthmetal, bromide of alkaline earth metal, and iodide of alkaline earthmetal.
 19. The method according to claim 1, wherein the vacuum is 1000Pa or less.